1. Field of the Invention
This invention relates generally to a resonant circuit and is directed more particularly to a high frequency wide band resonant circuit which is formed without using a distributed element and hence which can be easily made compact or small in size.
2. Description of the Prior Art
In order to select the signal received by a tuner of a receiver for a high frequency and wide band signal, for example, a UHF band television signal, a resonant circuit which uses a voltage controlled variable capacitance diode and a distributed element has been employed in many cases as disclosed in, for example, the U.S. Pat. No. 3,569,850.
In the prior art, the shape of the distributed element used in the resonant circuit is determined in compliance with the wave length at the resonant frequency, so that it is difficult to make the element compact. Further, since the distributed element is easily influenced by its environment and hence housed in a shielding case, it occupies a rather large space and its adjustment is difficult to be done. In addition, the inductance value of the distributed element is varied much by the resonant frequency which variation acts to cancel capacitance variation of the variable capacitance diode, so that a greater range of capacitance of the variable capacitance diode is required for a variation range of the resonant frequency than if the inductance of the distributed element were constant.
Further the voltage controlled variable capacitance diode is generally low in the quality factor Q and, in a UHF band, its resonant impedance becomes substantially equal to the impedance of an active element to which a resonant circuit containing the voltage controlled variable capacitance diode is connected (for example, the impedance between the gate and drain of an FET, and impedance between the base and collector of a bipolar transistor), so that signal transmission loss becomes great. Therefore, in the case where the voltage controlled variable capacitance diode is used in a tuner of a receiver for UHF band signals, it would be desirable to reduce the signal transmission loss as much as possible throughout the frequency band of the received signals.